Ever had a moment where you're researching something for hours, not being able to "get" what the person is saying, but then you listen to it being said differently and think "God, this is actually so fucking simple."? Afterwards, anything relevant to that new idea gets absorbed so much quicker, as if you'd known it all along.

Suppose you learned C. Afterwards, learning another imperative programming language is a piece of piss. You find that it could be a matter of just knowing the new syntax. Your understanding of the idea of C has allowed you to easily extrapolate it to a different means of implementation.

A line can easily be drawn between 'knowing' and 'not-knowing', but there isn't really a way to define an 'understanding'. You know it's there, but you can't really put it in words. It's a subjective and pretty ambiguous term. We wouldn't have all this technology without the ability to think in these abstractions.

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